[Python-Dev] PEP 567 pre v3 (original) (raw)

Yury Selivanov yselivanov.ml at gmail.com
Thu Jan 11 01:58:42 EST 2018

On Thu, Jan 11, 2018 at 10:35 AM, Chris Jerdonek <chris.jerdonek at gmail.com> wrote:

On Mon, Jan 8, 2018 at 11:02 PM, Nathaniel Smith <njs at pobox.com> wrote:

Right now, the set of valid states for a ContextVar are: it can hold any Python object, or it can be undefined. However, the only way it can be in the "undefined" state is in a new Context where it has never had a value; once it leaves the undefined state, it can never return to it. I know Yury responded to one aspect of this point later on in the thread. However, in terms of describing the possible states without reference to the internal Context mappings, IIUC, wouldn't it be more accurate to view a ContextVar as a stack of values rather than just the binary "holding an object or not"? This is to reflect the number of times set() has been called (and so the number of times reset() would need to be called to "empty" the ContextVar).

But why do you want to think of ContextVar as a stack of values? Or as something that is holding even one value?

Do Python variables hold/envelope objects they reference? No, they don't. They are simple names and are used to lookup objects in globals/locals dicts. ContextVars are very similar! They are keys in Context objects—that is it.

ContextVar.default is returned by ContextVar.get() when it cannot find the value for the context variable in the current Context object. If ContextVar.default was not provided, a LookupError is raised.

The reason why this is simpler for regular variables is because they have a dedicated syntax. Instead of writing

print(globals()['some_variable'])

we simply write

print(some_variable)

Similarly for context variables, we could have written:

print(copy_context()[var])

But instead we use a ContextVar.get():

print(var.get())

If we had a syntax support for context variables, it would be like this:

context var print(var) # Lookups 'var' in the current context

Although I very much doubt that we would ever want to have a dedicated syntax for context variables (they are very niche and are only needed in some very special cases), I hope that this line of thinking would help to clear the waters.

Yury

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